1. Field of the Invention
The present invention relates to a process for producing an improved water absorbing resin, and a water absorbing resin obtained by the process. More specifically, it relates to a process for producing an improved water absorbing resin by treating and crosslinking the vicinity of the surface of water absorbing resin particles with an aqueous solution of a specific water-soluble compound and a crosslinking agent, and a water absorbing resin obtained by the same process having a large absorbing ability in all circumstances, whether in a pressure-free state or in a pressurized state.
2. Description of the Prior Art
Hitherto, water absorbing resins have been widely used in hygienic materials such as sanitary materials and paper diapers, water retainers for soil, and others. Examples of such water absorbing resins are known to include crosslinked polyacrylic acid salts, self-crosslinking type polyacrylic acid salts, crosslinked copolymers of starch-grafted acrylic acid salts, copolymers of vinyl alcohol-acrylic acid salt, hydrolyzates of crosslinked copolymer of acrylamide, neutralized copolymers of crosslinked isobutylene-maleic anhydride, and crosslinked carboxy methyl cellulose salts.
The method for obtaining a water absorbing resin is mainly intended to make a water-soluble resin insoluble in water by slightly crosslinking. The crosslinking methods thereof include a method of adding a crosslinking agent when polymerizing a hydrophilic monomer and crosslinking simultaneously with polymerization, or a method of uniform crosslinking between molecules of water soluble polymers by reacting crosslinking agent to the water-soluble polymers.
The water absorbing resins obtained by these crosslinking methods have a minimized content of the crosslinking agent in order to enhance the absorbency. Accordingly, the water absorbing resin is weak in the gel strength and low in the absorbency under pressure. Moreover, inside the water absorbing resin, water-soluble components coexist, which are responsible for lowering the absorption rate, lowering the stability of the water absorbing gel by elution of water-soluble components after absorption, and deterioration of the dry feeling of water absorbing gel. On the other hand, the method of enhancing the crosslinking density by increasing the use of crosslinking agent is also known, but when uniformly crosslinked at high density, the absorption capacity, which is the basic feature of the water absorbing resin, is lowered, which is not preferable.
To solve these problems, as shown in items 1 to 4 below, methods for reforming the water absorbing resin particles have been proposed.
1 Method of dispersing a water absorbing resin in a large quantity of organic solvent such as alcohol, ketone and ether containing water, and crosslinking by adding a crosslinking agent (Japanese Patent Application Laid Open No. 44627/1982).
2 Method of crosslinking a water absorbing resin in water containing state with the water content adjusted to 10 to 40 wt. % (Japanese Patent Application Laid Open No. 62665/1984).
3 Method of absorbing crosslinking agent and water into water absorbing resin in the presence of inorganic powder to avoid the coagulation, and heating while stirring so as to crosslink and remove water simultaneously (Japanese Patent Application Laid Open No. 163956/1985).
4 Method of crosslinking by dispersing in a large quantity of hydrophilic inert solvent with boiling point of 100.degree. C. or higher, in the presence of 1.5 to 5.0 parts by weight of water and inert inorganic powder, to 1 part by weight of water absorbing resin (Japanese Patent Application Laid Open No. 147475/1985).
Of these reforming methods, however, in the method of 1, although a water absorbing resin with an improved absorption rate is obtained, a step for removing a large amount of the organic solvent having low boiling point is required, and it is hardly suited to industrial application. Furthermore, in this step of removing the organic solvent, the crosslinking agent permeates into the central part of the water absorbing resin particles together with the concentrated moisture so as to crosslink up to the inside of the resin particles, thereby lowering the absorption performance or mutually coagulating the water absorbing resin particles by means of the concentrated moisture to form lumps, and the working efficiency is poor.
In the method of 2, the crosslinking agent permeates into the central part of the water absorbing resin particles containing large amount of water to crosslink up to the inside of the resin particles, and the absorption performance is lowered, and the water absorbing resin particles containing water are mutually coagulated to form lumps.
In the methods of 3 and 4, the mutual coagulation of water absorbing resin particles may be somewhat prevented by the presence of the inorganic powder, but since the crosslinking agent is adsorbed not only by the water absorbing resin but also by the inorganic powder, crosslinking tends to become nonuniform, and sufficient performance may not be always exhibited. Furthermore, in the method of 4, since a large amount of water of 1.5 to 5.0 times of the water absorbing resin is used, the crosslinking agent may permeate into the central part of the water absorbing resin containing water, and the absorption performance is lowered.